Dissemination of tumor cells from primary tissue to distant organs with subsequent formation of secondary tumors is the major cause of mortality in men suffering from prostate cancer (CaP). In contrast to the primary tumor, metastasis is especially challenging to treat because of its systemic nature and frequent association with resistance to existing therapeutic agents. Monotherapies have failed at clinics because metastatic CaP is a multiple molecule-driven disease, making it important to identify a therapeutic regimen that would target key molecules in the pathogenesis of metastatic-CaP. There is an urgent need to identify molecules involved in metastasis that create opportunities to identify new therapeutic approaches to prevent or treat metastatic CaP. We have identified two key molecules (i) S100A4 and (ii) Rac1, which play role in the progression of disease from localized to metastatic CaP-phenotype. We provide evidence that S100A4 expression is increased during progressive stages of CaP in humans and transgenic mouse model, TRAMP and this protein regulates the migration of metastatic prostate tumor cells. Importantly, we found that targeting S100A4 could inhibit metastasis in TRAMP mice. The important information pertinent to this proposal is our data showing the efficacy of small molecule inhibitors of S100A4 in vitro and development of anti-S100A4 monoclonal antibodies (mAb-6B12 & 5c3-mAb) that has potential to inhibit metastasis in a mouse model. We also observed Rac1 activity leads to cytoskeleton instability and migration of primary prostate tumor cells. Based on these findings we generated our global hypothesis that targeting S100A4 and Rac1 simultaneously will be an ideal approach to prevent and treat locoregional growth and metastatic-CaP disease. We posit that S100A4-targeting agents (such as S100A4-inhibitor or anti-S100A4 antibody) in combination with Rac1-targeting agents (inhibitor) would inhibit the metastatic ability of both metastatic and potential invasive CaP cells. These hypotheses will be tested in the following three Aims: (aim#1). Determine the anti-metastatic efficacy of S100A4-inhibitor and anti- S100A4 antibodies (6B12 & 5C3 mAb)-based mono and combination therapies using cell-based tumor transendothelial and bone-marrow metastasis models of CaP disease. (aim#2). Determine the efficacy of the specific S100A4-inhibitor and anti-S100A4 antibody on the growth and metastasis of prostate tumor cells in orthotopic syngeneic and athymic mouse models (aim 3). Determine the efficacy of S100A4 and Rac1 targeted mono- and combination therapies in TRAMP mice, the autochthonous transgenic model of CaP-disease. Since very few options are available to treat metastatic-CaP disease, the successful outcome of this proposal will identify a new target-based approach to prevent and treat metastatic CaP disease in men.